A. Field of the Invention
The present invention relates to a method of stimulating a subterranean formation penetrated by a wellbore, particularly where a foamed acid is injected into the formation.
B. Description of the Prior Art
It is known to use foamed acid to treat subterranean formations penetrated by a wellbore. Representative of art teaching foamed acids for such use are Plummer et al., U.S. Pat. No. 3,100,528, which also teaches a cushion of inert gas may precede the foamed acid; Crawford et al., 15 Journal Petroleum Technology 237 (March, 1963), which teaches a foamed acid treatment using carbon dioxide; Neill et al., SPE Paper No. 738 (Oct. 6-9, 1963), which also teaches acidizing using carbon dioxide; Foshee et al., U.S. Pat. No. 3,323,593, which teaches compositions useful for fracturing and/or acidizing comprising foamed emulsions; Petty et al., U.S. Pat. No. 3,358,763, which teaches foamed acid treatments where nitrogen is added to the acid as a liquid, whereupon it vaporizes to form a foam; Jacobs et al., U.S. Pat. No. 3,330,346, which is specifically directed to foams using a particular class of foaming agents but which teaches such foams may be used in acidizing; Anderson et al., U.S. Pat. No. 3,612,179, which forms a foamed acid in situ by injecting in sequence, acid, an aqueous foaming agent, and a gas, and thereafter teaches to inject another slug of acid; Blauer et al., U.S. Pat. No. 3,937,283, which teaches optimum foam quality parameters; and Volz, U.S. Pat. No. 4,044,833, which teaches foamed acidizing wherein the foam contains a gelling agent.
Other possible pertinent art includes Plummer et al., U.S. Pat. No. 3,980,136, which is directed primarily to fracturing with foam including a partial bleedback step, but which suggests the fracturing foam may contain acid to stabilize clays; and Hutchinson et al., U.S. Pat. No. 3,463,231, and 3,572,440 which teach foamed acids, but apparently for use in cleaning the wellbore.
The teachings of all the foregoing art are expressly incorporated herein.
Notwithstanding the considerable body of art relating to foamed acid treatments, such treatments--particularly when carried out at rates and pressures sufficient to initiate or extend a fracture in the formation-have not proved as successful as might have been expected. Laboratory tests suggest that relatively high fluid loss may be the reason. Although the present invention is not to be limited as to any particular theory as to why fluid loss was relatively high in prior art foamed acid systems, it appears to be due to a combination of factors such as breaking of the foam upon reaction of the acid with the formation resulting in high calcium ion levels, loss of surfactant on the formation, loss of inert gas altering the foam quality to a point where the foam is no longer stable, and the like. Fluid loss of the foamed acid upon contact with the formation is significantly reduced according to the present invention.